In wireless communications, multi-path, fading, and other various interferences constitute major challenges to robust signal transmission. In the OFDMA system, the radio frequency spectrum may be regarded as a plane of two dimensions constituted by time intervals and sub-carriers. Each pair of the time interval and the sub-carrier forms a time-frequency grid which is a basic unit for signal transmission. After the transmission of the time-frequency grids, either channel responses or noise statistics or both may fluctuate. If symbols are transmitted over partial time-frequency grids with either greatly attenuated channel responses or large noise variance, the corresponding symbols may not be able to be recovered. It is unpractical to control power for each time-frequency grid because additional bandwidth is required.